1. Field of the Invention
The present invention is related to the field of transient electromagnetic field measurements made in a geological formation. Specifically, the invention increases an azimuthal sensitivity and resolution of the transient field to formation boundaries.
2. Description of the Related Art
Electromagnetic induction resistivity instruments can be used to determine the electrical conductivity of earth formations surrounding a wellbore. An electromagnetic induction well logging instrument is described, for example, in U.S. Pat. No. 5,452,761, to Beard et al. The instrument described in the Beard patent includes a transmitter coil and a plurality of receiver coils positioned at axially spaced apart locations along the instrument housing. An alternating current is passed through the transmitter coil. Voltages which are induced in the receiver coils as a result of alternating magnetic fields induced in the earth formations are then measured. The magnitude of certain phase components of the induced receiver voltages are related to the conductivity of the media surrounding the instrument.
The development of deep-looking electromagnetic tools has a long history. Such tools are used to achieve a variety of different objectives. Deep looking tools attempt to measure the reservoir properties between wells at distances ranging from tens to hundreds of meters (ultra-deep scale). There are single-well and cross-well approaches, most of which are rooted in the technologies of radar/seismic wave propagation physics. This group of tools is naturally limited by, among other things, their applicability to only high resistivity formations and the power available down-hole.
At the ultra-deep scale, technology may be employed based on transient field behavior. The transient electromagnetic (TEM) field method is widely used in surface geophysics. Examples of transient technology are seen, for example, in Kaufman et al., 1983, “Frequency and transient soundings”, Elsevier Science.; Sidorov et al., 1969, “Geophysical surveys with near zone transient EM.” published by NVIGG, Saratov, Russia; and Rabinovich et al., 1981, “Formation of an immersed vertical magnetic dipole field”: J. Geologiya I Geofizika, N 3. Typically, voltage or current pulses that are excited in a transmitter initiate the propagation of an electromagnetic signal in the earth formation. Electric currents diffuse outwards from the transmitter into the surrounding formation. At different times, information arrives at the measurement sensor from different investigation depths. Particularly, at a sufficiently late time, the transient electromagnetic field is sensitive only to remote formation zones and does not depend on the resistivity distribution in the vicinity of the transmitter (see Kaufman et al., 1983). This transient field is especially important for logging. Use of a symmetric logging tool using transient field measurements for formation detection is discussed, for example, in U.S. Pat. No. 5,530,359, to Habashy et al.
U.S. Pat. No. 5,955,884, to Payton et al., discusses methods for measuring transient electromagnetic fields in rock formations. Electromagnetic energy is applied to the formation and waveforms that maximize the radial depth of penetration of the magnetic and electric energy. Payton comprises at least one electromagnetic transmitter and at least one electric transmitter for applying electric energy. The transmitter may be either a single-axis or multi-axis electromagnetic and/or electric transmitter. In one embodiment the TEM transmitters and TEM receivers are separate modules that are spaced apart and interconnected by lengths of cable, with the TEM transmitter and TEM receiver modules being separated by an interval of from one meter up to 200 meters, as selected. Radial depth of investigation is related to the skin depth δ=√{square root over (2/σμω)} which in turn is related to frequency. Lower frequency signals can increase the skin depth. Similarly, the conductivity of the surrounding material inversely affects the skin depth. As conductivity increases, the depth of investigation decreases. Finite conductivity casing of the apparatus therefore can reduce the depth of investigation.
Rapidly emerging measurement-while-drilling (MWD) technology introduces a new, meso-deep (3–10 meters) scale for an electromagnetic logging application related to well navigation in thick reservoirs. A major problem associated with the MWD environment is the introduction of a metal drill pipe close to the area being measured. This pipe produces a very strong response and significantly reduces the sensitivity of the measured EM field to the effects of formation resistivities and remote boundaries. Previous solutions for this problem typically comprise creating a large spacing (up to 20 meters) between transmitter and receiver. Such a system is discussed in U.S. Pat. No. 6,188,222 B1, to Seydoux et al. The sensitivity of such a tool to remote boundaries is low. Currently, Stolar Horizon, Inc. is developing drill string radar (DSR) for Coal Bed Methane wells. DSR provides 3-D imaging within a close range of the wellbore.
Currently, induction tools operate to obtain measurements either in the presence of a primary field or by using transient field techniques. Examples of current techniques for obtaining measurements using either primary field or transient field phenomena in measurement-while-drilling methods include the Multiple Propagation Resistivity (MPR) device, and the High-Definition Induction Logging (HDIL) device for open hole that utilizes a transient technique. In these techniques, one or more transmitters disposed along a drill tool act as a primary source of induction, and signals are received from the formation at receiver coils placed at an axial distance from the transmitters along the drill tool. One disadvantage of both MPR and HDIL methods is that the primary source of induction from the transmitter is always present during the time frame in which the receivers are obtaining measurements from the formation, thereby distorting the intended signal. This can be solved by using pulse excitations such as is done in a transient induction tool.
In a typical transient induction tool, current in the transmitter coil drops from its initial value I0 to 0 at the moment t=0. Subsequent measurements are taken while the rotating tool is moving along the borehole trajectory. The currents induced in the drilling pipe and in the formation (i.e. eddy currents) begin diffusing from the region close to the transmitter coil in all the directions surrounding the transmitter. These currents induce electromagnetic field components which can be measured by induction coils placed along the conductive pipe. Signal contributions due to the eddy currents in the pipe are considered to be parasitic, since the signal due to these currents is much stronger than the signal from the formation. In order to receive a signal which is substantially unaffected by the eddy currents in the pipe, one can measure the signal at the very late stage, at a time in which the signals from the formation dominate parasitic signals due to the pipe. Although the formation signal dominates at the late stage, it is also very small, and reliable measurement can be difficult. In prior methods, increasing the distance between transmitter and receivers reduces the influence of the pipe and shifts dominant contribution of the formation to the earlier time range. Besides having limited resolution with respect to an oil/water boundary, such a system is very long (up to 10–15 m) which is not desirable and convenient for an MWD tool.
A number of publications describe different applications of a MPR resistivity logging measurements (see, for example, Meyer, W., 1997, Multi-parameter propagation resistivity interpretation, 38th SPWLA annual transactions, paper GG). All these publications describe dual pairs of transmitting antennas that permit long- and short-spaced measurements of phase difference and attenuation resistivities at the frequencies of 2 MHz and 400 MHz. The resulting resistivity curves support detailed quantitative and petrophysical analysis. Currently, the MPR tool has no means to resolve formation in azimuthal direction and the depth of investigation is limited to several feet.
MPR offers the benefits of several feet depth of investigation for Rt determination and bed boundary detection during reservoir navigation along with the enhanced accuracy over a broad range of resistivities. The lack of resolving capability in the azimuthal direction and inability to resolve ultra-deep formation represent the main limitation of MPR for geosteering. Indeed, in a formation such as FIG. 3A, the MPR tool has the same readings as there would be in the formation in FIG. 1B. Even a transversal arrangement of the transmitting and receiving coils such as in 3DEX does not distinguish between the model in FIG. 3A and the model in FIG. 3B.
U.S. patent application Ser. No. 10/295,969 of Tabarovsky discusses a method of obtaining a parameter of interest, such as resistivity, of an earth formation using a tool having a body with finite, non-zero conductivity. The method obtains a signal from the earth formation that is substantially independent of the conductivity of the tool. A first signal is produced using a transmitter on the tool. An axially separated receiver receives a second signal that results from an interaction of the first signal with the earth formation. The second signal is dependent on the conductivity of the induction tool. This second signal can be represented using a Taylor series expansion in one half of odd integer powers of time. At least one leading term of the Taylor series expansion can be subtracted from the second signal. By suitable processing of the signals, Tabarovsky teaches the determination of the formation resistivity. The examples given in the Tabarovsky application use z-oriented transmitter and receiver coils.
There is a need for increasing a sensitivity and resolution of measured transient fields in to a distant boundary in a geologic formation. The present invention fulfills this need.